Performance Evaluation of
Polybenzimidazole for Potential
Performance Evaluation of
Polybenzimidazole for Potential Aerospace
Applications
Proefschrift
ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus prof. ir. K.C.A.M. Luyben, voorzitter van het College voor Promoties,
in het openbaar te verdedigen op woensdag 15th of Januari 2014 om 15:00 uur
door
Hafiz Muhammad Saleem,
IQBAL
Hafiz Muhammad Saleem Iqbal geboren te Lahore, Pakistan
Dit proefschrift is goedgekeurd door de promotor:
Prof. Dr. ir. R. Benedictus Prof. Dr. S. Bhowmik
Samenstelling promotiecommissie:
Rector Magnificus, voorzitter
Prof. R. Benedictus, Technische Universiteit Delft, promotor Prof. Dr. S. Bhowmik, Amrita University, copromotor
Prof. dr. S.J. Picken, Technische Universiteit Delft Prof. dr. ir. R. Marissen, Technische Universiteit Delft Prof. dr.ir. H.E.J.G. Schlangen, Technische Universiteit Delft Dr. ir. K.M.B. Jansen, Technische Universiteit Delft Prof. Dr. Ir. R. Akkerman, Technische Universiteit Twente
ISBN: 978-90-8891-781-3
Keywords: Polybenzimidazole, Carbon Nano-fibers, Fire Testing, Radiation testing, Adhesive bonding, PBI coating, Environmental Testing
Copyrights @ 2014 by Hafiz M. Saleem Iqbal
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the author.
Dedicated To
My caring Parents
My loving wife
&
Summary
vii
SUMMARY
Performance Evaluation of Polybenzimidazole for
Potential Aerospace Applications
With the increasing use of polymer based composite materials, there is an increasing demand of polymeric resins with high glass transition temperature (Tg), high thermal stability and excellent mechanical properties at high temperature. Polybenzimidazole (PBI) is a recently emerged high performance polymer. It has the highest glass transition temperature (425°C) of any commercially available organic polymer, high decomposition temperature (500°C – 600°C), good oxidation resistance and it maintains excellent strength at cryogenic temperatures. Due to its excellent thermal and mechanical properties, PBI has great potential to be used for many high temperature applications. However, at the same time, it has very high melt viscosity which is too high to allow its processing by conventional manufacturing techniques.
The processing of PBI itself is a real challenge due to high processing temperature and pressure requirement. This is the main reason that compression molded PBI has found very few industrial applications. Recently, PBI is also available in solution form but its performance has not been explored for potential applications. In solution form, a great potential lies in exploring PBI as a film and coating material. Therefore, the objective of present work is to investigate the potential of PBI film and coating for various aerospace applications. The possible applications can be the thermal control films in spacecraft, protective coating for polymer composites in low earth orbit (LEO) and the radiation shielding material in geosynchronous earth orbit (GEO). The polymer also has the great prospective to be used as protective coating for composite aircraft. Due to its superior non-flammability, the material has a potential to be used as a fire resistant coating for aircraft application.
Summary
viii
In present work, unfilled and carbon nano-fibers (CNFs) reinforced PBI films were manufactured and their thermal and mechanical properties were investigated. Unfilled PBI film has exhibited thermal properties comparable to the thermal properties of molded PBI whereas it has demonstrated higher mechanical properties than molded PBI. Addition of 2 weight percent of CNFs has further improved the thermal and mechanical properties of PBI film.
In present work, performance of PBI is also evaluated as a fire resistant coating for aircraft. Unfilled PBI could not improve the fire resistant properties of aerospace grade unidirectional epoxy/carbon composite. However, addition of 2 weight percent of CNFs to PBI has improved the coating performance remarkably. Nano-filled PBI coating has reduced the average heat release rate (HRR) from 27 KW/m2 (for uncoated epoxy/carbon composite) to 1 kW/m2. Also, nano-filled PBI coating has reduced the peak heat release rate (PHHR) from 87 KW/m2 (for uncoated epoxy/carbon composite) to 53 KW/m2. Furthermore, ignition time of epoxy/carbon composite has increased from 165 seconds to 730 seconds. These results demonstrate the effectiveness and potential of PBI for fire resistant applications in aircraft.
Performance of both unfilled and nano-filled PBI films is evaluated after exposure to simulated LEO environment. Both unfilled and nano-filled PBI have retained most of thermal and mechanical properties after exposure to simulated LEO environmental conditions. Though, PBI has exhibited high erosion yield value which is a mass dependent parameter. However, a detailed investigation has shown that value of high erosion yield is due to the loss of water (absorbed by PBI) rather than the degradation of PBI itself.
Both unfilled PBI and nano-filled PBI were also exposed to different high energy radiations and they have retained most of their thermal and mechanical properties after exposure to these high energy radiations. Though, unfilled PBI has shown about 40% decrease in tensile strain after exposure to mixed radiations. However, addition of CNFs to PBI has improved its performance by retaining 80% tensile strain after exposure to mixed radiations. PBI has retained most of its properties after exposure to even higher doses of gamma radiations and electron radiations in present study. These results are encouraging and research can be further extended to evaluate the performance of
Summary
ix PBI after exposure to different radiations under various temperature conditions.
PBI is also evaluated as an adhesive for high temperature applications. Due to high solvent contents, the process optimization required lot of efforts to form PBI bonded joints with considerable lap shear strength. Therefore, in present work, efforts are devoted to optimize the adhesive bonding process of PBI. PBI adhesive bonded joints were formed successfully with single lap shear strength of 30 MPa using epoxy based unidirectional carbon fiber composite substrate. These results are quite encouraging and efforts can be made to further optimize the bonding process in order to improve the lap shear strength. PBI adhesive bonded joints were also exposed to hot/wet environment at 80oC and 95% RH for 1000 hours. Exposed joints have retained about 80% of the joint strength even after exposure to hot/wet environment for 1000 hours. Furthermore, PBI adhesive bonded joints were tested at 80oC to evaluate the high temperature performance of PBI. Bonded joints have retained about 50% of the original joint strength.
The present work has contributed to the knowledge and understanding of several aspects of the PBI polymer. Different problems related to the processing of PBI are highlighted and efforts are made to evaluate the performance of PBI as a film, coating and adhesive. PBI has not been manufactured and characterized in such detail in the past. Also the potential of nano-filler based PBI has never been explored in the past. Present work has produced some encouraging results to perform further research on PBI as a coating and adhesive material for different aerospace applications.
H.M.S.IQBAL Delft, Netherlands
Contents
xiContents
SUMMARY………..
vii
ABBREVIATIONS………..………..xix
Chapter 1 Introduction……….1
1.1. General Background………...………… 1
1.2. High Performance Polymers ... 2
1.2.1. Definition of High Performance Polymers ... 2
1.2.2. History of High Performance Polymers ... 2
1.3. Polybenzimidazole – A high Performance Polymer ... 3
1.3.1. Historical background of Polybenzimidazole (PBI) ... 4
1.3.2. Applications of Polybenzimidazole (PBI) ... 5
1.4. Research Goals ... 5
1.5. Outlines of Thesis ... 6
1.6. References ... 8
Chapter 2 Processing and Characterization of Unfilled and
Nano-fibers Reinforced Polybenzimidazole………
11
2.1. Introduction ... 11
2.2. Polymer Based Nano-composites ... 12
2.2.1. Processing issues of Carbon Nano-fibers ... 13
2.3. Experimental... 13
2.3.1. Materials ... 13
Contents
xii
2.3.3. Solution casting of Basic PBI Film ... 14
2.3.4. Solution casting of PBI nano-composite film ... 15
2.4. Thermal and Mechanical Characterization ... 15
2.4.1. Thermal Gravimetric Analysis (TGA) ... 15
2.4.2. Dynamic Mechanical Analysis (DMA) ... 16
2.4.3. Tensile Testing ... 16
2.4.4. Scanning Electron Microscopy (SEM Analysis) ... 16
2.5. Results and Discussion ... 16
2.5.1. Compression Molding of PBI ... 16
2.5.2. Processing of PBI neat film and nano-composite film ... 18
2.5.3. Thermal Gravimetric Analysis (TGA) ... 19
2.5.4. Dynamic Mechanical Analysis (DMA) ... 20
2.5.5. Tensile testing ... 24
2.5.6. Scanning Electron Microscope (SEM) Analysis ... 26
2.5.7. Fracture morphology ... 27
2.6. Conclusions ... 28
2.7. References ... 29
Chapter 3 Fire Testing of Nano-fibers Reinforced
Polybenzimidazole Coating for Aircraft Application…………..
31
3.1. Introduction ... 31
3.2. Combustion Process of Polymers ... 33
3.3. Classification of Polymers Based on Thermal Decomposition
Mechanism ... 35
3.4. Effect of Char Formation on Fire Resistant Properties of Polymers
36
3.5. Elements of Material Flammability ... 37
3.5.1. Time-to-Ignition (TTI) ... 37
Contents
xiii
3.5.3. Flame Propagation Index (FPI) ... 38
3.5.4. Smoke generation ... 38
3.6. Methods to Improve the Fire Resistance of Polymers and Polymer
Based Composites... 39
3.6.1. Addition of flame retardants to Polymers ... 39
3.6.2. Limitation of Using Fire retardants ... 40
3.6.3. Nano-filler based flame retardant nano-composites ... 40
3.6.4. Application of fire protective coatings ... 41
3.7. Experimental... 43
3.7.1. Materials ... 43
3.7.2. Preparation of Composite Laminates ... 43
3.7.3. Plasma Treatment of Composite Laminates ... 43
3.7.4. Contact Angle Measurement ... 43
3.7.5. Adhesion Testing ... 43
3.7.6. Preparation of Unfilled PBI Coated Samples ... 44
3.7.7. Preparation of Nano-filled PBI Coated Samples ... 44
3.7.8. Thermal gravemetric Analysis (TGA) ... 44
3.7.9. Cone Calorimeter and fire testing ... 45
3.8. Results and Discussion ... 45
3.8.1. Thermal Gravimetric Analysis (TGA) ... 45
3.8.2. Contact Angle measurement of Plasma treated Composite
Substrate ... 47
3.8.3. Adhesion Testing ... 48
3.8.4. Cone Calorimeter test results ... 50
3.9. Conclusions ... 58
3.10. References ... 60
Chapter 4 Performance Evaluation of Polybenzimidazole
in Simulated Low Earth Orbit Environment………....
63
Contents
xiv
4.2. LEO Space Environment ... 64
4.2.1. Ultra High Vacuum ... 64
4.2.2. Ultraviolet radiations (UV radiations) ... 64
4.2.3. Thermal Cycling ... 65
4.2.4. Atomic Oxygen (AO) ... 66
4.3. Experimental... 67
4.3.1. Materials ... 67
4.3.2. Solution casting of Unfilled PBI Film ... 67
4.3.3. Solution casting of nano-filled PBI films ... 67
4.3.4. Exposure to LEO Environment ... 67
4.3.5. Tensile Testing ... 70
4.3.6. Scanning Electron Microscopy (SEM) ... 70
4.3.7. Atomic Force Microscopy (AFM) ... 70
4.4. Results and Discussion ... 70
4.4.1. Mass Loss and Erosion Yield Measurements of exposed samples 70
4.4.2. Tensile Test Results ... 73
4.4.3. Scanning Electron Microscopy (SEM) ... 77
4.4.4. Atomic Force Microscopy (AFM) ... 80
4.5. Conclusion ... 82
4.6. References ... 82
Chapter 5 Performance Evaluation of Polybenzimidazole
under High Energy Radiations Environment………....
85
5.1. Introduction ... 85
5.2. Effect of Ionizing Radiations on Polymeric Materials ... 86
5.3. Experimental... 87
5.3.1. Materials ... 87
5.3.2. Preparation of Unfilled PBI and Nano-filled PBI Film ... 87
5.3.3. Exposure to Electron radiations and Gamma radiations ... 87
Contents
xv
5.4. Testing and Characterization ... 88
5.4.1. Thermal Gravimetric Analysis (TGA) ... 88
5.4.2. Dynamic Mechanical Analysis (DMA) ... 88
5.4.3. Tensile Testing ... 89
5.4.4. UV/VIS Spectroscopy ... 89
5.4.5. Surface morphology ... 89
5.5. Results and Discussion ... 89
5.5.1. Thermal gravimetric analysis (TGA) ... 89
5.5.2. Dynamic Mechanical Analysis (DMA) ... 92
5.5.3. Tensile Test Results ... 96
5.5.4. UV/VIS photospectroscopy ... 98
5.5.5. Surface morphology ... 101
5.6. Conclusion ... 103
5.7. References ... 104
Chapter 6 Performance Evaluation of Polybenzimidazole
as an Adhesive for Aerospace Applications……….
107
6.1. Introduction ... 107
6.2. Selection of the surface treatment ... 109
6.2.1. Atmospheric Pressure Plasma Treatment (APPT) ... 110
6.3. Experimental... 110
6.3.1. Materials ... 110
6.3.2. Atmospheric Pressure Plasma Treatment of Composite Specimen
………..111
6.3.3. Contact Angle Measurements ... 111
6.3.4. Specimen Preparation for Lap Shear Testing ... 112
6.3.5. Environmental Conditioning of Specimens ... 112
6.3.6. Specimen Preparation for Lap Shear Testing ... 112
6.3.7. Scanning Electron Microscopy (SEM) ... 113
Contents
xvi
6.4.1. Atmospheric Pressure Plasma Treatment ... 113
6.4.2. Surface Treatment for Lap Shear Testing... 120
6.4.3. PBI adhesive bonded joints Preparation ... 121
6.4.4. Process Optimization of PBI Adhesive to form bonded joints ... 121
6.4.5. Environmental Conditioning of PBI adhesive bonded joints ... 128
6.4.6. Testing of PBI adhesive bonded Joints at High Temperature ... 131
6.4.7. Study of Failure Modes ... 133
6.5. Conclusions ... 135
6.6. References ... 136
Chapter 7 Performance Evaluation of Polybenzimidazole
coating for Aircraft Application……….………
139
7.1. Introduction ... 139
7.2. Experimental... 140
7.2.1. Materials ... 140
7.3. Testing and Characterization ... 141
7.3.1. Atmospheric pressure Plasma Treatment of Composite Substrate
………. 141
7.3.2. Contact Angle Measurements ... 141
7.3.3. Application of Coating ... 142
7.3.4. Adhesion Testing of PBI coating ... 142
7.4. Environmental Resistance of PBI coating ... 142
7.4.1. Conditioning of PBI coated Panels at high Temperature and
Humidity... 142
7.4.2. Liquid Immersion of PBI Coated Panels ... 142
7.4.3. Adhesion testing of Conditioned Samples ... 143
7.4.4. Scratch testing of PBI Coated panels after exposure to Various
Environmental Conditions ... 143
Contents
xvii
7.4.5. Nano-indentation testing of PBI Coated panels after exposure to
Various Environmental Conditions ... 143
7.5. Results and Discussion ... 144
7.5.1. Contact angle measurement of Atmospheric Pressure Plasma
Treated Composite Substrate ... 144
7.5.2. Adhesion Testing of PBI coating ... 145
7.5.3. Adhesion testing of Conditioned Specimens ... 147
7.5.4. Scratch Testing of PBI coated Panels after exposing to Various
Environmental Conditions ... 148
7.5.5. Nano-indentation testing of PBI coated Panels after exposing to
Various Environmental Conditions ... 153
7.6. Conclusions ... 156
7.7. References ... 157
Conclusions and Recommendations for Future Work……..
159
8.1. Conclusions ... 159
8.1.1. Compression Molding of PBI ... 159
8.1.2. Solution Casting of PBI film ... 160
8.1.3. Thermal and Mechanical Properties of PBI ... 161
8.1.4. Testing in LEO Simulated Environment ... 161
8.1.5. Radiation Testing ... 162
8.1.6. Fire Testing of PBI Coating ... 163
8.1.7. Lap Shear Testing of PBI ... 163
8.1.8. Testing for Aircraft Application ... 164
8.2. Recommendations for future work ... 165
CURRICULUM VITAE……….……….
170
LIST OF PUBLICATIONS
………...171
Abbreviations
xixABBREVIATIONS
PBI Polybenzimidazole DMAc Dimethyl-acetamide CNFs Carbon Nano-fibersAPPT Atmospheric Pressure Plasma Treatment
TGA Thermal Gravimetric Analysis
DMA Dynamic Mechanical Analysis
SEM Scanning Electron Microscope
HRR Heat Release Rate
PHRR Peak Heat Release Rate
TTI Time-to-Ignition
SEA Smoke Extinction Area
MLR Mass loss rate
LEO Low Earth Orbit
GEO Geosynchronous Earth Orbit
UV Ultraviolet
AO Atomic Oxygen
AFM Atomic force microscopy
CA Contact Angle
1
C
HAPTER
1
Introduction
In this chapter, a brief introduction about Polybenzimidazole and its historical background is presented. Motivations behind this thesis and its research objectives are described. The structure of this dissertation is also outlined briefly.
1.1. General Background
Polymers and polymer based composite materials have been gaining significant attention since last decade. With wide range of possible fillers and resins, composite materials have opened the way to an enormous range of materials with diverse chemical, physical, and mechanical properties. These materials have an impressive and diverse range of applications in automotive, aviation, spacecrafts, civil infrastructure and sports industry [1-5]. In recent times, there has been a steady increase in the use of polymer based composite materials in both military and commercial aircraft [3, 6-7]. Polymeric materials are also being extensively used in space systems as multi-layer insulations, matrix in the substrate for solar panels, adhesives, thermal control coatings for spacecraft and insulation for electrical wiring [8, 9]. Despite having many advantages, polymeric based materials are facing challenges for demanding aerospace applications. As an example, at a temperature of above 200oC, many of the polymeric materials cannot maintain their thermal stability and mechanical properties which ultimately leads to collapse of the composite structures [10]. Also, the exposure of these materials to harsh space environment, affects the thermal, mechanical and optical properties of these materials [9, 11-12]. These demanding conditions are limiting the use of polymer based materials for aerospace industry.
Introduction
2
Therefore, in order to meet the requirement of aerospace industry, research is continued to develop new high performance polymers which can maintain the desired properties under working environmental conditions for longer duration of time and serve their purpose properly.
1.2. High Performance Polymers
1.2.1.
Definition of High Performance Polymers
High performance polymers can be defined in many ways depending on the type of application. High performance is a general term which can be used to describe many polymers. In term of high temperature applications, the most popular definition of high performance polymer is the polymer which can retain the useable properties at a temperature greater than 177oC [13]. These polymers have a glass transition (Tg) greater than 200oC and they can maintain their thermal stability (5% weight loss) up to a temperature of 450oC [13].
1.2.2.
History of High Performance Polymers
High performance polymers have long historical background. The development of high performance polymers started in late 1950s mainly to meet the requirement of aerospace industry. The most productive period for the development of high performance polymers was between 1960 and 1970. During this period, many high performance polymers were developed with high glass transition temperature (Tg) and high thermal stability. Despite their high thermal stability, cost and processing of these polymers was the major issue. The chemical structure of these high performance polymers resulted in difficult processing and poor solubility in organic solvents. Their softening points or melt viscosities were too high to process using conventional manufacturing techniques. Polybenzimidazole, polyimide and polysulfone are the main examples of initially developed high performance polymers. Polybenzimidazole could not get the attraction to a sizeable market due to its high cost and difficult processability [13, 14]. Polyimide (PI) film designated as Kapton was the first material among high performance polymers which was fabricated by DuPont. Kapton attracted a big market and since then it has been the largest selling high performance polymer in the world. In aerospace industry, it is used primarily as insulation for aircraft and spacecraft wiring. Other applications include high performance flexible films for photovoltaic substrates and printed circuit boards. After the commercialization of
Chapter 1
3 Kapton in 1970s, efforts were made to develop other high performance polymers with low cost, easy processing and improved mechanical properties. One example of such polymer is thermoplastic Polyetherimide (PEI) designated as Ultem produced by General Electric [13]. Kapton is also being used in satellites for thermal management [15]. Other important developments during this period were polyamide imide (PAI) and polybenzoxazole (PBO) but they never reached the success of PI.
1.3. Polybenzimidazole – A high Performance
Polymer
One of the families of high performance polymers is the imidized family. Imidized polymers are said to offer the best combination of thermal and mechanical properties of any commercially available polymer. The family of imidized polymers includes polyimide (PI), polyamideimide (PAI), polybenzimidazole (PBI), and polyetherimide (PEI). Other than PI from imidized family, PBI has gained wide recognition in recent years as high-temperature polymer. PBI is a thermoplastic polymer and, in molded form, it has the highest compressive and tensile strength of any unfilled polymeric resin [16-18]. PBI has the highest glass transition temperature (425°C) of any commercial available organic polymer [19]. It has high decomposition temperatures (500°C-600°C), good oxidation resistance and it maintains excellent strength at cryogenic temperatures [20]. The chemical structure of PBI is shown in figure 1.1
Figure 1.1 Chemical Structure of PBI
PBI has both aromatic and heterocyclic rings in its backbone structure. Aromatic rings are hydrocarbons which contain benzene (C6H6). Benzene is often drawn as a ring of six carbon atoms, with alternating double bonds and single bonds. Chemical structure of benzene is shown in figure 1.2.
Introduction
4
Figure 1.2 Chemical Structure of Benzene (Aromatic Ring)
Aromatic rings which contain non-carbon atoms (Oxygen, Nitrogen or Sulfur) in the ring are called heterocyclic rings. The examples of heterocyclic rings are shown in figure 1.3.
Imidazole Benzimidazole
Figure 1.3 Examples of Heterocyclic Rings
Because of these aromatic and heterocyclic rings in the backbone structure of PBI, it exhibits high thermal stability and resistance to oxidative decomposition [21]. Due to its excellent thermal and mechanical properties, PBI can be one of the leading candidates for high temperature aerospace applications.
1.3.1.
Historical background of Polybenzimidazole (PBI)
Polybenzimidazole is regarded as the first high performance polymer which was developed in early 1960s. But this polymer could not get the attraction of sizeable market due to high cost and difficult processing [13, 14]. Due to the unique characteristics of PBI polymer, efforts were continued to develop some techniques for the processing of this polymer. It was not until mid 1980s that compression molded PBI parts were manufactured successfully by Hoechst Celanese and Alpha Precision Plastics [14]. The marketing of molded PBI was started with the trade name Celazole. Alpha took over the technology of Celazole completely in 1995 and continued marketing of molded PBI parts for sealing elements in high-temperature corrosive environments. Currently, PBI is finding a lot more applications in many diverse fields in the form of fabric, molded parts and thin films.
Chapter 1
5
1.3.2.
Applications of Polybenzimidazole (PBI)
PBI is claimed to have better strength than PI and PAI at higher temperature and superior in terms of chemical and radiation resistance [17, 22]. It has shown considerable promise, not only as a matrix for high-temperature structural composite materials, but also as fibers [23]. Automotive and aerospace applications of PBI include thermal insulators, high-performance bearings, electrical connectors, nose cone of aircraft and ablative structures [17, 24]. Due to its superior non-flammability, PBI has been used for firefighters’ protective clothing, high-temperature gloves, and astronaut flight suits [25-28]. In recent years, acid doped PBI has emerged as a promising material for the application in the membrane of the fuel cell [28-29].
1.4. Research Goals
Inspired by the excellent thermal and mechanical properties of PBI, the aim of this work is to explore the potential of PBI as a film, coating and adhesive material for different aerospace applications. Another direction which has not been explored yet is the manufacturing of nano-composite PBI. Therefore, another objective of this work is to manufacture the carbon nano-fibers (CNFs) reinforced PBI film and coating and to study the effect of CNFs on thermal and mechanical properties of PBI. Considering the high thermal and mechanical properties of PBI, it has great potential to be used for various aerospace applications. The possible applications are the film for thermal management in spacecraft, protective coating for polymer composite in LEO environment and the radiation shielding material in GEO environment. The polymer also has great perspective to be used as protective coating for composite aircraft. Due to its superior non-flammability, this material must be evaluated as a fire resistant coating for aircraft application. Based on above mentioned objectives, the most important questions to be addressed in this work are:
a. What are the issues that may be encountered during the processing of PBI molded parts, unfilled PBI film and CNFs reinforced PBI film?
b. In solution form, PBI can be used as coating material. However, it contains about 75% of the solvent in the solution and it requires a temperature of around 200oC to extract the maximum solvent out of it. But this temperature may not be achieved while
Introduction
6
applying PBI coating on composite substrate due to the fact that properties of most of the composites degrade at high temperature. This fact will make it challenging to apply PBI coating on composite substrate. Therefore, the question need to be addressed that if PBI coating can be applied on the composite substrate at low curing temperature? And if the application of PBI coating is possible at low temperature then to what extent it will affect the performance of PBI?
c. PBI has superior non-flammability in molded form. However, it has never been tested as a fire resistant coating. Inspired by its thermal properties, one of the objectives of this work is to evaluate that if PBI has a real potential to be used as a fire resistant coating for epoxy based carbon fiber composite?
d. Space environment consist of ultraviolet radiations, ionizing radiations, high vacuum, thermal cycling. These radiations and thermal cycling degrade the thermo-mechanical and optical properties of polymers. In this context, the question need to be addressed is; how and to what extent, the exposure of PBI to simulated Low Earth Orbit environment and under high energy radiations, will affect the thermo-mechanical and optical properties of PBI.
This work only includes the initial evaluation of PBI for different aerospace applications. Detail evaluation of the material for any specific application is out of the scope of this work. However, recommendations for the future work will be given on the basis of the results obtained in this work. On the basis of these recommendations, PBI can be further evaluated in detail for a specific application.
1.5. Outlines of Thesis
This thesis mainly divided into three parts. Part 1 includes the process optimization, thermal and mechanical characterization of unfilled and nano-fibers reinforced PBI. Part 2 mainly relates to the evaluation of unfilled and nano-filler based PBI film and coating materials for potential aerospace applications. Part 3 is related to the evaluation of PBI as an adhesive for high temperature applications. A separate chapter is included for the conclusions and recommendation for future work.
Chapter 1
7 A brief history of high performance polymers and their development with the course of time is presented in the introductory chapter. The development history of Polybenzimidazole (PBI) and its thermal and mechanical properties are presented in this chapter. Current state of art applications and potential future applications of PBI are highlighted in this chapter.
Chapter 2 explains the manufacturing of both compression molded PBI and PBI film. Different techniques are illustrated to manufacture the CNFs based PBI nano-composite. Thermal and mechanical properties of PBI in different forms are presented in this chapter. SEM analysis of fractured surfaces of tested specimens is also presented in this chapter. Chapter 3 deals with the study and evaluation of PBI as a fire resistant coating for epoxy based unidirectional carbon fiber composite. A comparison of fire resistant properties of uncoated carbon/epoxy composite and composite coated with unfilled and nano-filled PBI coating is presented in this chapter.
Chapter 4 and chapter 5 mainly deals with the exposure of basic PBI and PBI nano-composite to different space environmental conditions and its evaluation after having exposed to these conditions. The effect of simulated LEO environment and high energy radiations on characteristic behavior of unfilled PBI and PBI nano-composite is assessed and results are presented to evaluate its potential for future space applications.
Chapter 6 shed light whether or not it is feasible to fabricate the bonded joints of polymer based composite material for aerospace application using PBI as an adhesive. On the basis of the results obtained, the potential of PBI as an adhesive for high temperature applications is also discussed.
Chapter 7 studies the behavior of PBI as a protective coating for epoxy based carbon fiber composite for aircraft application. Response of PBI is described after exposure to hot wet environment and liquid immersion. Critical properties including hardness, scratch resistance and adhesion of PBI coating are assessed and the results are presented.
Introduction
8
Chapter 8 summarizes the undertaken research and highlights the main findings of this work. On the basis of these finding, some conclusions are made and recommendations are given for future work.
1.6. References
1- A. Baker, S. Dutton, K. Donald, Composite materials for aircraft structures, 2nd edition, 2004
2- A.P. Mouritz, E. Gellert, P. Burchill, K. Challis, Review of advanced composite structures for naval ships and submarines, Composite structures 53 (2001)
3- A.P.Mouritz and A.G.Gibson, Fire Properties of polymer composite materials, Solid Mechanics and its applications, Volume 143, 2006, ISBN-13 978-1-4020-5356-6
4- A.P.Mouritz, Fire safety of advanced composites for aircraft, B2004/0046, April 2006
5- E. Grossman, I. Gouzman, Space Environment Effects on Polymers in Low Earth Orbit, Nuclear Instruments and Methods in Physics Research B 208, 2003
6- A.P. Mouritz, Fire resistance of aircraft composite laminates, Journal of Material Science Letters, 22, (2203), pp. 1507-1509
7- S. Gandhi, Postcrash Health Hazards from Burning Aircraft Composites, Galaxy Scientific Corporation, Fire Safety Section, AAR-422, Federal Aviation Administration
8- L. R. Kiefer and A. R. Orwoll, space environmental effects on polymeric materials, final technical report n88- 168 79, 1987
9- E. M. Silverman, Space environmental effects on spacecraft: LEO materials selection guide, NASA Contractor Report 4661, Part 2
10- S. Feih, A. P. Mouritz, Z. Mathys and A.G. Gibson, Fire Structural Modeling of Polymer Composites with Passive Thermal Barrier, Journal of Fire Sciences, Vol. 28 – March 2010
11- Joo-Hyun Han, Chun-Gon Kim, Low Earth Orbit Space Environment Simulation and its Effects On Graphite/Epoxy Composites, Composite Structures vol. 72, 2006
12- Kwang-Bok Shina, Chun-Gon Kima, Chang-Sun Honga, Ho-Hyung Leeb, Prediction of Failure Thermal Cycles in Graphite/Epoxy Composite Materials Under Simulated Low Earth Orbit Environments, Composites: Part B, Vol. 31, 2000
Chapter 1
9
13- Paul M. Hergenrother, The Use, Design, Synthesis, and Properties of High Performance/High Temperature Polymers: An Overview, 2003
14- Tai-Shung Chung, A Critical Review of Polybenzimidazoles , Department of
Chemical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Republic of Singapore, 119260, Available online: 23 Sep 2006
15- A. N. Hammoud, E. D. Baumann, E. Overton, I. T. Myers, J. L. Suthar, W. Khachen and J. R. Laghari, High Temperature Dielectric Properties of Apical, Kapton, Peek, Teflon AF, And Upilex Polymers, Conference on Electrical Insulation and Dielectric Phenomenon, USA, 1992, 549-554
16- Tai-Shung Chung, A Critical Review of Polybenzimidazoles , Department of Chemical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Republic of Singapore, 119260, Available online: 23 Sep 2006
17- Kemmish, D. J., “High Performance Engineering Plastics,” Rapra Review Reports, Vol. 8, No. 2, 1995, Rept. 86.
18- Beland, S., High Performance Thermoplastic Resins and Their Composites, 1st ed., Noyes Data Corp., Park Ridge, NJ, 1990, pp. 47–50
19- Steinerand, P. A., and Sandor, R., “Polybenzimidazole Prepreg: Improved Elevated Temperature Properties with Autoclave Processability,” High Performance Polymers,Vol. 3, No. 3, 1991, pp. 139–150.
20- Bhowmik, S., Bonin, H.W., Bui, V. T., and Weir, R. D., “Modification of High-Performance Polymer Composite Through High-Energy Radiation and Low-Pressure Plasma for Aerospace and Space Applications,” Journal of Applied Polymer Science, Vol. 102, Jan. 2006, pp. 1959–1967.
21- O. R. Hughes, O. N. Chen, W.M. Cooper, L. P. Disano, E. Alvarez and T.E. Andres, PBI powder processing to performance parts, Journal of Applied Polymer Science, Published online 10 March 2003, Vol. 53, No. 5,1994, pp. 485- 496 22- Yasuhiko Onishi, Atsushi Maeno, Hideyuki Tanigawa, Kazuo bra,Takashi Ito,
Masayuki Nagata and Tsuneo Sasuga, Poly( benzimidazole) as insulator for superconductors, Cryogenics 35 ( 1995) 795-798
23- “Fibrous Reinforcement for Space Applications,” NASA CR-796, 1967.
24- Robert R. Dickey a, John H. Lundell a & John A. Parker, The Development of
Polybenzimidazole Composites as Ablative Shields, NASA-Ames Research Center, Moffett Field
25- Zhang, L., Ni, Q.-Q., Shiga, A., Fu, Y., and Natsuki, T., “Synthesis and Mechanical Properties of Polybenzimidazole Nanocomposites Reinforced byVapor Grown Carbon Nanofibers,” Polymer Composites, Vol. 31, No. 3, 2009, pp. 491–496.
Introduction
10
26- Okamoto, M., Fujigaya, T., and Nakashima, N., “Individual Dissolution of Single-Walled Carbon Nanotubes by Using Polybenzimidazole, and Highly Effective Reinforcement of Their Composite Films,” Advanced Functional Materials, Vol. 18, No. 12, 2008, pp. 1776–1782.
27- Sidman, K. R., and Gregory, J. B., “Development of Thermally Stable Polybenzimidazole (PBI) Fiber,” Aeronautical Systems Div., TR 72-50, Nov. 1971.
28- Sannigrahi, A., Arunbabu, D., Sankar, M., and Jana, T., “Tuning the Molecular Properties of Polybenzimidazole by Copolymerization,” Journal of Physical Chemistry B, Vol. 111, No. 42, 2007, pp. 12124–12132.
29- Hel, R., Li, Q., Bach, A., Jensen, J. O., and Bjerrum, N. J., “Physicochemical
Properties of Phosphoric Acid Doped Polybenzimidazole Membranes for Fuel Cells,” Journal of Membrane Science, Vol. 277, 2006, pp. 38–45.
11
C
HAPTER
2
Processing and Characterization of Unfilled and
Nano-fibers Reinforced Polybenzimidazole
In this chapter, processing of Polybenzimidazole is presented both in powder and solution form. Processing of nano-fibers reinforced PBI films is also discussed. Thermal and mechanical properties of compression molded PBI, unfilled PBI film and carbon nano-fibers reinforced PBI films were evaluated using Thermal-Gravimetric Analysis, Dynamic Mechanical Analysis and tensile testing and results are presented.
2.1. Introduction
Polybenzimidazole (PBI) is a thermoplastic polymer having excellent thermo-mechanical and chemical properties [1]. In molded form, PBI has the highest compressive and tensile strength of any unfilled polymeric resin [2, 3]. It has the highest glass transition temperature (425°C) of any commercial available organic polymer [1]. It exhibits high decomposition temperatures (500°C-600°C), good oxidation resistance and it maintains excellent strength at cryogenic temperatures [4]. Despite having better thermal and mechanical properties, the real challenge is the processing of PBI as such, because this particular polymer is not melt processable. Moreover, compression molded PBI requires very high processing temperature (440°C) and pressure (50 MPa). Therefore, very few research articles are available till today regarding processing of PBI. Some researchers have demonstrated compression molding processing of PBI using PBI powder whereas only two articles have demonstrated the processing of PBI in solution form to produce PBI nano-composite films [5-7]. However, a
Processing and Characterization of Unfilled and Nano-fibers Reinforced PBI
12
detailed study about thermal and mechanical behavior of unfilled PBI and carbon nano-fibers reinforced PBI nano-composite under different environmental conditions is not conducted till date. In this context, present chapter will cover the processing of PBI using PBI powder and PBI solution. A detailed study about thermal ad mechanical behavior of unfilled and nano-filled PBI will also be the part of this chapter.
2.2. Polymer Based Nano-composites
Nano-composites are a class of composite materials where one of the constituents has dimensions in the range of nanometers [8, 9]. In the last decade, there has been a strong emphasis on the development of polymeric nano-composites. The increasing interest in the development of polymeric nano-composites exists not only because of their high specific strength but also for the possibility of making products with unique mechanical, electrical and thermal properties [10, 11]. However these properties depend on several factors such as type of nano-filler, surface treatments, type of polymer matrix and synthesis methods [12]. At present, there are many types of nano-fillers available to produce polymer nano-composites. Some of the most commonly used nano-fillers are carbon nano-fibers (CNFs), carbon nano-tubes (CNTs) and nano-clay. CNFs and CNTs have been used to modify several polymers including polypropylene, polymethyl-methacrylate, polyethylene-terepthalate, polystyrene and polycarbonate [13]. Both CNFs and CNTs have similar rope-like structures, but CNTs exhibits much smaller diameter and better mechanical, thermal and electrical properties compared to CNFs [11, 14]. However, addition of CNTs to polymeric resin significantly increases the viscosity of polymers. In addition, it is difficult to uniformly disperse CNTs into polymer due to strong Vander Waals forces between them. Contrary to CNTs, CNFs are drawing significant attention to form nano-composite because of relatively better dispersion and low cost [10-11, 15]. The potential of nano-filler reinforced PBI nano-composite has not been explored yet. Considering the high thermal and mechanical properties of PBI, CNFs based PBI nano-composite has great potential to be used for various aerospace applications. Therefore, the objective of this work is to manufacture CNFs reinforced PBI film and coating in order to study the effect of nano-fibers on thermal and mechanical properties of PBI. CNFs were selected as nano-filler due to the ease of dispersion and relatively low cost as compare to CNTs.
Chapter 2
13
2.2.1.
Processing issues of Carbon Nano-fibers
Dispersion of CNFs into polymer matrix is not an easy task. There are numbers of challenges arise from the small size of CNFs during the manufacturing process. Although significant advances are made in recent years to overcome the problems of dispersing CNFs, processing still remains a key challenge in fully utilizing the properties of these fibers. A primary difficulty while adding the nano-filler to the polymer is to attain a good dispersion of the nano-filler, independent of filler shape and aspect ratio. Without proper dispersion, filler agglomerations tend to act as defect sites which limit the mechanical performance of the nano-composite. Therefore, it is very important to get the uniform dispersion of CNFs in the polymer matrix to take the full advantage of nano-fibers reinforcement. Different dispersion methods are being used including the dilution method, mechanical stirring, bath sonication, melt spinning, extrusion processing and high shear mixing [16, 17]. Researchers have obtained well dispersed carbon nano-fillers in a polymer by high energy sonication of polymer solution containing dispersed nano-fillers followed by a solvent-evaporation method which successfully achieved homogeneous composites [17, 18].
2.3. Experimental
2.3.1.
Materials
PBI powder having molecular weight of 20,000 g/mole was supplied by PBI Performance Products. 26% concentrated solution of Polybenzimidazole (PBI) in Dimethyl-acetamide (DMAc) was supplied by PBI Performance Products. This solution was used for casting the film of unfilled PBI and CNFs reinforced PBI. CNFs with diameter ranging from 70 nm to 200 nm and length 50 μm to 200 μm were supplied by Pyrograf Products, Inc. with trade name PR-19-XT-LHT.
2.3.2.
Processing of Polybenzimidazole by Compression Molding
Processing of PBI powder was carried out using compression molding. As PBI is not melt processable, therefore, during compression molding process, the polymer is heated above its glass transition temperature. The pressure was varied during compression molding in order to get specimens with high tensile properties. Following are the three different methods that were used to make the specimens using PBI powder.
Processing and Characterization of Unfilled and Nano-fibers Reinforced PBI
14
Method 1
In first method, mold was filled with required amount of PBI powder and placed in the press. Mold was heated to 440°C at a rate of 5°C/min. It was kept at 440°C for 30 minutes to get a uniform heat distribution followed by applying a pressure of 10 MPa. The mold was kept at this pressure and temperature for 3 hours. After 3 hours, the mold was allowed to cool down to temperature of 50°C. Afterwards, pressure was released and the molded part was removed.
Method 2
In second method, mold was filled with required amount of PBI powder and placed in the press. Mold was heated to 440°C at a rate of 5°C/min. It was kept at 440°C for 30 minutes to get a uniform heat distribution followed by applying a pressure of 50 MPa. The mold was kept at this pressure and temperature for 3 hours. After 3 hours, the mold was allowed to cool down to a temperature of 50°C. Afterwards, pressure was released and the molded part was removed. The difference between first and second method is the change of pressure during compression molding. The pressure has increased from 10 MPa to 50 MPa in the second method.
Method 3
In this method, PBI powder was dried at 200°C for 12 hours in forced air convection oven. After drying the PBI powder, mold was filled and placed in the press. Mold was heated to 440°C at a rate of 5°C/min. It was kept at 440°C for 30 minutes to get a uniform heat distribution. Then a pressure of 50 MPa was applied on the mold with same temperature for 3 hours. During this period, the pressure was released for 30 seconds in order to escape any gas evolved during heating process. After 3 hours, the mold was allowed to cool down to a temperature of 50°C. At this point, the pressure was released and the molded part was removed.
2.3.3.
Solution casting of Basic PBI Film
In present work, efforts were made to fabricate PBI films using PBI solution in DMAc. The as-received solution of PBI in DMAc was highly viscous and therefore, it was diluted to 17% by adding DMAc followed by mechanical stirring at 60°C for 15 minutes. The mixture was then used to produce 60 to 80 µm thick film. The film was prepared by spreading the mixture over the glass plate with the help of adjustable
Chapter 2
15 doctor blade. The film was allowed to dry in the vacuum oven at 80°C for 2 hours and then it was further heated at 200°C for overnight. Then the film was peeled off from the glass plate by immersing in the hot distilled water. After removing the film from hot distilled water, wrinkles were formed in the film more likely due the presence of some DMAc and moisture. Therefore, in order to remove the wrinkles before mechanical testing, the film was pressed in the hot press at 250°C and at a pressure of 0.5 MPa.
2.3.4.
Solution casting of PBI nano-composite film
In present work, PBI nano-composite film was prepared using mechanical stirring and bath sonication methods. In first method, pre-calculated amount of CNFs was carefully weighed and directly mixed in the diluted solution of PBI. The mixture was then stirred mechanically using IKA stirrer at 300 rpm for 30 minutes. In another method, calculated amount of CNFs was weighed and added to the DMAc solvent. CNFs were dispersed in DMAc solvent using bath sonication for 30 minutes at 60°C. After bath sonication of CNFs in DMAc, they were added to PBI solution. The sonication process in combination with mechanical stirring was continued for next 15 minutes. The mixture was then used to cast the film on the glass plate as described in section 2.3.3. The nano-composite films were prepared with CNFs contents of 0.5 wt%, 1 wt% and 2 wt%.
2.4.
Thermal and Mechanical Characterization
2.4.1.
Thermal Gravimetric Analysis (TGA)
Thermal gravimetric analysis (TGA) was conducted to determine the thermal stability of PBI powder, compression molded PBI, PBI neat film and PBI nano-composite film. Tests were performed using Perkins Elmer Thermal Analysis Instrument (Pyris Diamond Thermal Gravimetric Analyzer). Compression molded samples were cut into small pieces using a cutter to get the small size specimens for TGA test. The weight of all the samples was maintained between 5 to 10 mg. The samples were heated from a temperature range of 25°C to 550°C at a heating rate of 10°C/min. The furnace was purged with nitrogen gas to prevent oxidation at a flow rate of 25ml/min.
Processing and Characterization of Unfilled and Nano-fibers Reinforced PBI
16
2.4.2.
Dynamic Mechanical Analysis (DMA)
Dynamic mechanical analysis (DMA) was performed in tensile mode at an oscillation frequency of 1 Hz using Perkin-Elmer dynamic mechanical analyzer (Pyris Dynamic Mechanical Analyzer). Samples of compression molded PBI were cut into rectangular shape to final dimensions of (40x8x1.2) mm3. PBI neat film and nano-composite films were cut into rectangular shape having dimension of (40x8x0.06) mm3. Data was collected from 25oC to 500oC at a scanning rate of 3oC/min. Elastic modulus and loss factor of the all the samples were measured.
2.4.3.
Tensile Testing
Tensile testing of compression molded PBI, unfilled PBI film and nano-composite films was carried out using Zwick Tensile machine at a test speed of 2 mm/min at 20°C. Rectangular specimens of 100x10x0.06 mm3 were cut from the casted films. Compression molded specimens were machined to Dumbbell-shape according to ISO-527 for tensile testing. Five specimens for each material were tested for the reproducibility of the results. Force-displacement curves were recorded from which Young’s Modulus and tensile strength was evaluated. An extensometer was also used to determine the exact value of Young’s Modulus.
2.4.4.
Scanning Electron Microscopy (SEM Analysis)
Scanning electron microscopy of gold coated films was carried out to examine the dispersion of CNFs in the polymer matrix and to study the fracture surfaces after mechanical testing. Images were obtained using JOEL-7500-FE scanning electron microscope.
2.5.
Results and Discussion
2.5.1.
Compression Molding of PBI
Compression molding process was used to fabricate molded parts of PBI using PBI powder. The aim was to manufacture the specimens with high tensile strength. Three different methods were adapted to manufacture the specimen using compression molding process. In first method, mold was heated to 440°C followed by applying a pressure of 10 MPa. The mold was kept at this pressure and temperature for 3 hours. The specimen obtained after compression molding is shown in figure 2.1a. The specimen was not fully compacted and ultimately
Chapter 2
17 broken while removing from the mold. Failure in getting proper specimen is more likely due to the low pressure during compression molding. This low pressure resulted in incomplete compactness of the material. Required temperature and pressure was not applied to get enough compaction of the material so that it can get desired strength.
Figure 2.1 (a) Molded specimen compressed at 10MPa and 440°C (b) Molded specimen compressed at 50 MPa and 440°C (c) Molded specimen compressed at 50 MPa and 440°C using pre-dried PBI powder
In the second method, molding pressure was increased from 10 MPa to 50 MPa. The mold was heated to 440°C followed by applying a pressure of 50 MPa at 440°C. The mold was kept at this pressure and temperature for 3 hours. The specimen obtained after compression molding is shown in figure 2.1b. This time the specimen got enough compaction and specimen with desired shape is obtained. However, C-Scan of this specimen depicted some voids in the specimen which was more likely due to the fact that PBI powder was not dried before performing the compression molding process. Due to the presence of moisture, the molded specimen got voids inside which ultimately resulted in decreased tensile strength. The results of tensile tests are presented in coming section.
(a)
(b)
Processing and Characterization of Unfilled and Nano-fibers Reinforced PBI
18
In order to prevent voids in the molded specimen, PBI powder was dried at 200°C for 12 hours in forced air convection oven. After drying the PBI powder, compression molding process was used to form PBI molded parts using a temperature of 440°C and molding pressure of 50 MPa. The specimen obtained with this method is shown in figure 2.1c. Using this method, proper specimen was obtained having no voids.
2.5.2.
Processing of PBI neat film and nano-composite film
PBI films were produced to evaluate thermal and mechanical properties of the polymer film. Unfilled and CNFs reinforced PBI films obtained after solution casting is shown in figure 2.2.
Figure 2.2 (a) PBI neat film (b) PBI with 2 wt. % of CNF obtained after direct mechanical mixing (c) PBI with 2 wt. % of CNF obtained after bath sonication of CNFs into DMAc solvent followed by mechanical mixing (d) Compression molded PBI sample
Unfilled PBI film is shown in figure 2.2a whereas, PBI nano-composite films obtained after performing two different dispersion techniques are shown in figure 2.2b and 2.2c. The nano-composite film obtained with separate dispersion of CNFs in DMAc solvent using bath sonication is shown in figure 2.2c. After dispersion of CNFs in the polymer solution, the color of the film was totally changed from brown, as in the case of unfilled PBI film, to blackish which could be the first test of examining the uniform dispersion of CNFs in the polymer matrix. PBI nano-composite film obtained by directing mixing of CNFs into the PBI resin has shown agglomeration which is clearly evident from figure 2.2b.
(a)
(d)
(c)
Chapter 2
19 These results indicate that direct mixing of CNFs into PBI resin is not an effective way to prepare nano-fibers reinforced PBI. Therefore, it can be concluded that bath sonication of CNFs before adding to PBI solution is more effective way to get PBI nano-composite films. Compression molded specimen before mechanical testing is shown in figure 2.4d.
2.5.3.
Thermal Gravimetric Analysis (TGA)
Thermal Gravimetric Analysis (TGA) of PBI powder, compression molded PBI, PBI neat film and PBI nano-composite film was carried out to determine the thermal stability of PBI. Figure 2.3a represents a comparison of thermal stability of PBI in different forms.
Figure 2.3 Comparison of thermal stability of (a) PBI in different forms (b) Comparison of thermal stability of unfilled PBI and nano-filled PBI film with 0.5 wt%, 1wt% and 2wt% of CNFs
TGA curve of PBI powder has shown two steps decomposition. PBI has shown initial weight loss of 2.35% which occurred between 40°C and 130°C. This initial weight loss is more likely due to the evaporation of water. The weight loss at the initial stage can be minimized by drying the powder in the oven. After first degradation step, weight loss starts very slowly but continuously until the temperature reached to 550°C. PBI has depicted only 5% weight loss up to this temperature. The high thermal stability of PBI is due to the presence of aromatic and heterocyclic ring in the polymer chain. The second degradation step started around 500°C. The weight loss around 500°C is more likely due to the loss of phenol and other gaseous products which is confirmed by experiments [19]. Due to high thermal stability, PBI has real potential to be used for high temperature applications. Comparing the thermal decomposition behavior of compression molded PBI with PBI powder, it
52 62 72 82 92 102 0 100 200 300 400 500 600 W e ig h t (%) Temperature (°C) PBI-Powder PBI-Molded PBI Film a b c a b c
(a)
52 62 72 82 92 102 0 100 200 300 400 500 600 W e ig h t (%) Temperature (°C) Unfilled PBI PBI+0.5%CNF PBI+1%CNF PBI+2%CNF a b d c a b c d(b)
Processing and Characterization of Unfilled and Nano-fibers Reinforced PBI
20
is observed that compression molded PBI has high thermal stability at initial stage up to a temperature range of 200°C. The high thermal stability of compression molded specimen is more likely because of its exposure to the high temperature during compression molding process. High temperature has dried the material and removes moisture which was present in PBI powder. On the other hand, for compression molded PBI, the weight loss occurred continuously up to a temperature of 325°C without any stable plateau. After this temperature, the polymer exhibits high thermal stability and only 0.2% weight loss occurred from a temperature of 400°C to a temperature of 550°C. Compression molded PBI has depicted a total weight loss of 6.5% up to the temperature of 550°C which is the indication of high thermal stability of any polymer at this temperature.
TGA curve for unfilled PBI film and carbon nano-fibers reinforced PBI film is shown in figure 2.3b. TGA curve for unfilled PBI film shows two steps degradation. The first degradation step started around 50°C and continued up to 150°C with a total weight loss of 9%. The weight loss is more likely due to absorption of moisture by the polymer and any DMAc remained inside the polymer film. After first degradation step, the plateau of TGA curve remained stable and polymer has shown only 10% total weight loss up to the temperature of 400°C. Afterwards, second degradation step started and unfilled PBI has depicted a total weight loss of 13% up to the temperature of 550°C. Addition of 0.5 wt. % and 1wt % of CNFs to PBI has improved the thermal stability of PBI. Addition of CNFs to PBI has reduced the first degradation step and improved the overall thermal stability of polymer. After addition of CNFs to PBI, it has depicted a total weight loss of 8% up to the temperature of 550°C. Furthermore, addition of 2 weight % of CNFs to PBI has almost eliminated the first degradation step. These results indicate that addition of CNFs to PBI has reduced the moisture absorption ability of PBI.
2.5.4.
Dynamic Mechanical Analysis (DMA)
Dynamical Mechanical Analysis (DMA) is performed in tensile mode at a fixed frequency of 1Hz to study the storage modulus (E’) as a function of temperature and to determine the glass transition temperature (Tg). A comparison of storage modulus of compression molded PBI and PBI film is shown in figure 2.4a.
Chapter 2
21 Figure 2.4 Comparison of (a) storage modulus and (b) loss factor curve of molded PBI and PBI film
Figure 2.4a reveals that compression molded specimen has higher value of storage modulus throughout the temperature range when compared to the storage modulus of PBI film. One of the possible explanations of low storage modulus of PBI film is the DMAc solvent which could be present in the film. The presence of DMAc solvent could have an effect on the stiffness of PBI film. The decrease in storage modulus after the first plateau is typically related to the small movement of polymeric molecules in the side chains [20]. This is the typical characteristic of an amorphous polymer which shows two plateaus in the storage modulus and then there is a sharp decrease in storage modulus. The sharp decrease in storage modulus after second plateau is due to the large movements of the molecules in the polymer backbone which cause the polymer to get softens and thus a decrease in stiffness of the polymer is observed [20].
When compared the loss factor curves of both compression molded PBI and PBI film, it is observed that compression molded PBI has a glass transition temperature (Tg) around 430°C. This Tg is higher than the Tg of unfilled PBI film as shown in figure 2.4b. On the other hand, PBI film has a wider loss factor curve which indicates the better cross linking of PBI film.
A comparison of storage modulus of unfilled PBI film and CNFs reinforced PBI films is shown in figure 2.5a. The figure demonstrates that addition of 0.5 wt. % of CNFs has increased the storage modulus up to 88% (from 4.2 to 8 GPa). By further adding CNFs in polymer up to 1 wt. %, a small decrease in storage modulus is observed but this value is still higher than storage modulus of unfilled PBI film. An
1E+06 1E+07 1E+08 1E+09 1E+10 0 100 200 300 400 500 600 St or ag e M od u lu s (E' ) Temperature (°C) PBI-Molded PBI Film a b a b
!["A Ptolemaic Inscription from Thera", P. M. Fraser, "The Journal of Egyptian Archeology", vol. 44, 1958 : [recenzja]](data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==)